The present invention relates to desiccants based on modified zeolites having an isotherm separation factor within the range of from about 0.07 to about 0.1 and their use in gas-fired cooling and dehumidification equipment.
Desiccant cooling systems employ an open cycle to process water vapor between a conditioned space and the environment. The use of thermal energy creates a chemical potential that can be used to produce a cooling effect. If the air can be made dry enough, an evaporative cooler will produce air that is as cold as a conventional electric air conditioner. The overall performance of these systems relies on the quality of the thermal energy input (availability) and the environment as both a cold sink and as a source of chemical potential (unsaturated air). Most solid desiccant cooling cycles consist of a desiccant dehumidifier, a sensible heat exchanger and two evaporative coolers. There are two important modes of operation: 1) The ventilation mode where outdoor air is processed to produce low enthalpy air for the cooled space, and 2) The recirculation mode where air from the cooled space is processed to maintain low enthalpy air conditions in the space.
A schematic of the ventilation mode (a) and a psychrometric representation of the cycle (b) are shown in FIG. 1. Ambient air at (1) is adiabatically dehumidified by the desiccant (DH). The hot, dry air at (2) is cooled by the sensible heat exchanger (HE) to create dry cool air at (3). This air is then adiabatically humidified by the evaporative cooler (EC) to produce cold, nearly saturated air at (4) that enters the building. Simultaneously, an equal amount of building air at (6) is adiabatically humidified to (7). This produces the cold sink for the dry air (3). The air is then heated by the same heat exchanger to (8). This is an attempt to recuperate as much of the heat of sorption as possible from the dehumidification process. The enthalpy of the air must now be increased to (9) by a thermal energy input such as natural gas heating. This air is then passed through the desiccant in order to regenerate it. The warm, humid air that exits the desiccant at (10) is then exhausted to the atmosphere.
A schematic of the recirculation mode (a) and a psychrometric representation of the cycle (b) are shown in FIG. 2. This time ambient air at (1) is adiabatically humidified to (2). This air now becomes the cold sink for the cycle. It is heated by the sensible heat exchanger to (3) and then further heated to (4) by an external thermal input such as natural gas heating. This air is then used to regenerate the desiccant material. The warm, humid air that exits the desiccant at (5) is returned to the environment. Simultaneously, room air at (6) is adiabatically dehumidified by the desiccant to produce warm, dry air at (7). This air is then cooled with the humidified and cooled ambient air to create cool, dry air at (8). The air is then adiabatically humidified to produce the cold, near saturated air at (9) which is returned to the building.
Both of the operational modes previously described operate on a continuous basis. That is, both the dehumidification and the regeneration processes occur at different parts of the cycle simultaneously. In order to accomplish this, the desiccant is deployed into a rotating wheel or drum that continuously cycles the desiccant between the dehumidification and regeneration air streams. The heat exchanger may be of a rotating or static design and the evaporative coolers must be distinctly separate units.
Three important interrelated performance parameters that determine the viability of desiccant cooling systems include:
1. Thermal coefficient of performance (COP).
The thermal COP is the nondimensional ratio of the amount of cooling output that is produced by a given amount of external energy input. For comparative purposes, COP is often quoted at Air Conditioning and Refrigeration Institute (ARI) rating conditions of 95.degree. F. dry bulb and 75.degree. F. wet bulb outdoor temperatures and 80.degree. F. dry bulb and 67.degree. wet bulb indoor temperatures. PA1 The EER is a measure of the amount of cooling, in thermal units (BTUs), that is produced by a given amount of parasitic electric energy input for fans, pumps, etc. in electrical units (Watts). PA1 This factor is defined as tons of cooling capacity per 1000 cubic feet per minute of supply air.
2. Parasitic electric energy efficiency ratio (EER).
3. Specific cooling capacity (SCC).
A combination of higher COP values and increased SCC values yields desiccant cooling systems that are more efficient and cost effective than state of the art devices. It has been determined that one of the primary factors affecting these values is the identity of the particular desiccant material employed in the system in terms of its psychrometric performance in the dehumidifier section of the system. For both the dehumidification and regeneration processes, there are two fundamental wavefronts that occur. The first and fastest wave is primarily a thermal front that is most affected by the total amount of thermal heat capacity associated with the dehumidifier. The second and slowest wave is the main concentration wavefront with strong associated thermal effects. Without getting into the details of the physical chemistry involved, it has been postulated that the primary function of an ideal desiccant material in an open cycle desiccant cooling system should be to produce the sharpest possible concentration wavefronts for both the dehumidification and regeneration processes.
Properties of ideal desiccants effecting these characteristics have been determined to include low heat of adsorption, high water adsorption capacity, high diffusivity of water, high chemical and physical stability towards heat, and most importantly, the shape of the desiccant isotherm.
It has been determined that the ideal shape of the desiccant isotherm for use in gas fired cooling/dehumidifier systems has a separation factor ranging from 0.07 to about 0.1 in accordance with the isotherm equation: ##EQU1## wherein x is the normalized loading fraction of water, P is the relative vapor pressure of water and R is the separation factor.
Several adsorption isotherms with different identified separation factors are shown in FIG. 3. Extreme Brunauer Type I isotherm is shown by the curve designated 0.01, whereas linear and less than linear isotherms are shown by the line designated 1.0 and the curve designated 10.0. The ideal isotherm shape is depicted as the area between the curves designated 0.07 and 0.1.
Most of the commercially available desiccant materials have not been developed for the specific purpose of providing space cooling. In most present day applications, the necessity of achieving efficient regeneration as well as deep drying of the air has not been a consideration. The requirement of attaining the very sharp adsorption wavefronts associated with molecular sieves along with the more efficient regeneration characteristics of the desiccant are what makes this application unique commercially. It is not surprising therefore, that none of the commercially available desiccants match the properties needed. The isotherms of various commercial and laboratory-developed desiccants are shown in FIG. 4. The trend is to see linear or nearly linear (Brunauer Type II) isotherms or extreme (Brunauer Type I) isotherms, as compared with the nearly ideal isotherm shape (moderate type I or Langmuir) designated as the "desired shape" for the purposes of this invention. This isotherm has a separation factor (R) of about 0.1.
Zeolites, both natural and synthetic, have been demonstrated in the past to have sorbent capabilities for water. Zeolites are crystalline, hydrated aluminosilicates with three-dimensional framework structure. The aluminosilicate framework is built up such that they possess cavities and channels of various dimensions depending upon the type of zeolite. In the structure of zeolite, Al.sup.3+ substitutes for Si.sup.4+, and hence develops a net negative charge which is balanced by different alkali metal and alkaline earth cations. It has been established that the charge-balancing cations of one type can be replaced by another (ion exchange), in most cases, without changing the crystalline structure. Because of small pore size, and presence of negative charge due to Al.sup.3+ substitution and alkali cations in the cavities, zeolites have a large affinity towards water molecules. The net effect is to exhibit an extreme Type I (Langmuir) adsorption isotherm with water as illustrated in FIGS. 3 and 4. For a constant polarity on the surface of porous solids, the effect of reduced pore size is to give an enhanced adsorption at low relative pressure (extreme Langmuir type isotherm) due to an overlapping of potential fields from the neighboring walls of the pore. On the other hand, for a constant pore sized material, increased polarity on the pore surfaces gives an enhanced adsorption of water at low relative pressures and vice versa. For zeolites, ion exchange with various alkali cations effectively reduces the volume of readsorption in accordance with the size of the ions, but not the affinity between zeolite and water molecules provided the structure is not distorted by ion exchange. The effect is again to exhibit an extreme Type I isotherm as illustrated in FIG. 3.
It is known in the prior art that Zeolite materials may be dealuminized to increase the ratio of silica to aluminum and may also be subjected to ion exchange reactions to replace the charge-balancing metal ions with protons, i.e., hydrogen ions.
For example, U.S. Pat No. 4,740,292 discloses a dealuminization process comprising reacting a faujasite type zeolite or zeolite beta with a strong mineral acid or organic acid at temperatures up to boiling to extract aluminum. The reference also indicates that the process also at least partially replaces metal ions present in the crystalline structure with protons. The purpose of the treatment is to render mixtures of the zeolites more effective as catalytic cracking catalysts for hydrocarbon feedstocks.
U.S. Pat. No. 4,701,431 teaches a process for dealuminization of zeolite, such as zeolite Y, by treatment with ethylenediaminetetracetic acid or a derivative thereof to dealuminize the zeolite, followed by an ion exchange reaction wherein the zeolite cations are at least partially replaced with rare earth metal cations. Once again the treatment is said to render those materials suitable as catalysts for various chemical processes or as sorbents.
Similar processes are disclosed in U.S. Pat. Nos. 3,551,353 and 4,477,336.
U.S. Pat. No. 3,140,251 discloses a process for enhancing the catalytic activity of aluminosilicates comprising reacting the aluminosilicate with an ammonium compound as a source of ammonium ions, followed by heat treatment of the resulting complex to decompose the ammonium complex to provide the hydrogen ion form of the zeolite.
While these and other prior art disclosures generally recognize that the properties of zeolite materials may be tailored by altering the chemical composition to render them more effective in given catalytic or molecular sieve applications, none of these disclosures has an objective of altering zeolite materials to render them more suitable for use as desiccant materials in gas fired cooling and air conditioning applications or the achievement of ideal desiccant materials for such applications having an isotherm separation factor within the range of from about 0.07 to about 0.1.
Accordingly, it is an object of this invention to provide zeolite materials which have been chemically modified to achieve an isotherm with a separation factor range of from about 0.07 to about 0.1.
Another object of this invention is to provide modified zeolite materials which are ideally suited for use as desiccants in gas fired open space air-conditioning and dehumidification systems.